With reference to FIG. 1, a ducted fan gas turbine engine generally indicated at 10 has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate-pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, and intermediate-pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.
The gas turbine engine 10 works in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate-pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate-pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate-pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.
Electrical power is usually extracted from such an engine for use within the aircraft by a wound-field synchronous generator. The generator can be mechanically connected to either the high-pressure shaft or to the intermediate-pressure shaft, via a transmission drive and accessory gearbox. DC current is applied to the rotor of the generator in the field winding. The frequency of the current produced in the generator stator winding is thus directly proportional to the speed of the shaft to which the generator is connected, the gear ratio between the engine shaft and the generator, and the number of pole pairs in the generator.
In contemporary aircraft platforms, the output frequency range of the generator typically varies over a frequency range of 400 to 800 Hz; the exact numbers depending upon the platform, and corresponding directly to an acceptable speed range for the shaft to which the generator is connected. The generator frequency range is provided to the suppliers of electrical equipment within the aircraft, so that their equipment can be configured to receive voltage in this frequency range.
Due to its variable output frequency, this type of generator is known as variable frequency starter generator (VFSG). FIG. 2 shows a schematic diagram of a VFSG, which includes a permanent magnet alternator (PMA) 30, main exciter 31 and main generator 32. The rotating parts of the PMA, exciter and generator are physically all mounted on the same shaft 33 and rotate at the same speed. The DC current injected into the field winding of the main generator comes from a rotating diode rectifier which is powered from the main exciter, which in turn is powered from the PMA.
The PMA 30 has permanent magnets mounted on its rotor 34. As the rotor spins, an AC main exciter voltage is induced across the stationary armature winding 35 of the PMA. This winding is connected to a voltage regulator circuit 36 which rectifies a controlled amount of AC current from the PMA stator winding and injects DC current into the stationary field winding 37 of the exciter 31. This in turn induces an AC voltage across the rotating armature winding 38 of the exciter (the exciter is referred to as inside out, with a stationary field winding and a rotating armature winding). A rotating diode rectifier circuit 39, producing a DC current, is connected to the armature winding of the exciter. The output of this rotating rectifier is then connected to the rotating field winding 40 of the main generator 32, inducing a controlled, AC voltage across the generator's stationary armature winding 41. Due to the high speed of rotation of the shaft 33 in aerospace generators, a brushed system for applying field current to the rotor of the main generator is not desirable.
The voltage regulator circuit 36 responds to changes in the load on the generator to maintain its output voltage at the required magnitude. It does not affect the generator output frequency. In some arrangements, for example during starting, the PMA 30 may not be used and electrical power can be provided directly to the exciter 31 from an alternative source.
The use of a VFSG and direct mechanical coupling between the engine shaft and the generator means that a restriction on the frequency range of the generator electrical output maps directly to a speed range restriction on the engine shaft. The ratio of maximum to minimum speed is typically around 2.2:1 (producing a frequency range of e.g. 800 Hz to 360 Hz).
If the VFSG and gearbox are configured to produce maximum frequency when the engine shaft is at its maximum speed, the minimum frequency condition effectively imposes a minimum speed and therefore a minimum thrust condition on the engine. During idling conditions, such as descent and taxiing, it is desired that the engine should produce as little thrust as possible, in order to save fuel. However, in order to remain within the electrical frequency range of the generator, the idle thrust of the engine may have to be set artificially high. Therefore it is desirable to have some degree of freedom between the electrical frequency and the mechanical speed.
It would be desirable to provide a generator which can vary the frequency of its output current independently of engine speed.